System and apparatus for obtaining a beam of high energy electrons from charged particle accelerators



June Z,- 1953 H. c. POLLOCK v 2,640,923 SYSTEM AND APPARATUS FOROBTAINING A BEAM OF HIGH ENERGY ELECTRONS FROM CHARGED PARTICLEACCELERATORS Filed March 31, 1950 4 Sheets-Sheet 1 Inventor: Herbert G.Pollock His Attorney.

June 2, 1953 H. c. POLLOCK 2,640,923

SYSTEM AND APPARATUS FOR OBTAINING A BEAM OF HIGH ENERGY ELECTRONS' FROMCHARGED PARTICLE ACCELERATORS Filed March 31, 1950 4 Sheets-Sheet '2 20/- POLE FflCE 0 I I 20 Fig. 4

I 34 la 2e- 55 39 44 i i I 5 ,38 4/ DISC/MI? 7055 swlrcfl SOUKffF I I 32E L 3] I Inventor Herbert C. ollock Hi5 Attorney June 2, 1953 c, POLLOCK2,640,923

SYSTEM AND APPARATUS FOR OBTAINING A'BEAM OF HIGH ENERGY ELECTRONS FROM.CHARGED PARTICLE ACCELERATORS Filed March 31, 1950 4 Sheets-Sheet Z5Invefitor: Herbert G. Pollock,

His Attorney.

June'Z, 1953 H. c. POLLOCK 2,640,923

SYSTEM AND APPARATUS FOR OBTAINING A BEAM OF HIGH ENERGY ELECTRONS FROMCHARGED PARTICLE ACCELERATORS Filed March 31, 1950 4 Sheets-Sheet 4DISTflNGE OF MAXIMUM BEAM INTENSITY FROM LEG.

EXTRA CTOI VOLTA 6E SOURCE OF 1Qw C. 3/

Inventor": HePbertQPo l loc k,

H s Attorney.

Patented June 2, 1953 SYSTEM AND APPARATUS FOR OBTAINING A BEAM OF HIGHENERGY ELECTRONS FROM CHARGED PARTICLE ACCELERA- TORS Herbert C.Pollock, Schenectady, N. Y., assig'nor to General Electric Company, acorporation of New York Application March 31, 1950, Serial No. 153,241

Claims. (01. 25027) The present invention relates to systems, andapparatus for obtaining a beam of high energy electrons from a chargedparticle accelerator.

It is known that high energy may be imparted to charged particles suchas electrons by accelerating the particles in an orbital path. Practicaland highly efficient apparatus for accomplishing such a result isdisclosed in U. S. Patent 2,485,409, granted October 18, 1949, toHerbert C. Pollock and Willem F. Westendorp, and assigned to theassignee of this present invention. This apparatus comprises means forinitially accelerating charged particles, usually electrons, in anoribital path by the action of a time-varying magnetic flux which linksthe orbital path to accelerate the electrons and a time-varying magneticguide field which traverses the path to constrain the electrons thereto.After the electrons have reached a velocity near that of light,additional energy is imparted to them by a localized electric field ofcyclically varying character. With such apparatus, it is possible toobtain electron energy levels of several hundred million electron volts.

Although such apparatus as that described in the above-mentioned patenthas been outstandingly successful in the acceleration of electrons tohigh energy levels in an orbital path, additional apparatus is desirableto facilitate the direct utilization of the high energy electrons.Commonly, the energy of the accelerated electrons is utilized by causingthe electrons to impinge upon a suitable target within the acceleratorapparatus whereby X-rays of correspondingly high intensity, which may bedirected from the accelerator apparatus for suitable application, willbe generated. For some applications, however, it is desirable to extractthe electrons themselves from the accelerator apparatus, rather thanemploying them first to generate X-rays before beneficial use isrealized. Accordingly, a principal object of the present invention is toprovide systems and apparatus for the practical and efilcient productionof high energy electron beams which may be utilized external to theaccelerator apparatus.

One aspect of the invention exemplary of the principles thereof morefully described and defined hereinafter comprises injecting electronsinto an orbital path enclosed by an evacuated annular vessel, initiallyaccelerating the electrons in the orbital path by the action of a.time-varying magnetic flux which links the path to acceleratetheelectrons and a time-varying magnetic guide field which traverses thepath to constrain the electrons thereto, imparting additional energy tothe electrons by coupling a cyclically varying electric field to theelectrons within the orbital path, diverting the electrons from theorbital path into spiral paths after a desired energy level has beenobtained, increasing the pitch of the spiral paths followed by theelectrons by causing the electrons to traverse a second electric field,the intensity of which is proportional in time to the intensity of theaforesaid magnetic guide field, and ejecting the electrons in a beamfrom the evacuated vessel by directing them through an electronpermeable window in the vessel.

The features of the invention desired to be protected herein are pointedout with particularity in the appended claims. The invention itself,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the following accompanying drawings in which Fig. 1 is a partiallysectionalized elevation of accelerator apparatus suitably embodying theinvention; Fig. 2 is a graphical representation helpful in explainingthe invention; Fig. 3 is a second graphical representation useful inexplaining the invention; Fig. 4 is a schematic representation ofexcitation equipment useful in connection with the device of Fig. 1;Fig. 5 is an enlarged detail view of one portion of the elevation ofFig. 1; Fig. 6

is a cross section taken on line 66 of Fig. 5; Fig." '7 is a crosssection taken on line l''l of Fig. 5;

Fig.8 is a schematic representation showing the location of elementswithin the accelerator apparatus of the invention; Fig. 9 is a thirdgraphical representation usefulin explaining the invention; Fig. 10 is aschematic representation of alternative circuitry which may be employedin connection with the excitation equipment represented in Fig. 4; Fig.11 is a schematic representation of other alternative circuitry whichmay be employed in connection with the excitation equipment representedin Fig. 4; Fig. 12 is an elevation of structure which may be employed inconnection with the invention as an alternative to that shown in Fig. 5;and Fig. 13 is a cross section taken on line |3l3 of Fig. 12.

Referring particularly to Fig. 1, there is shown in section a closed,rotationally symmetrical envelope [0, of suitable dielectric materialsuch as glass or porcelain, which defines within its interior an annularchamber. The envelope I0 provides a circular orbit in which electronsderived from a suitably energized source II supported from a side arm l2may be accelerated to a high energy level. The envelope III ispreferably highly evacuated and is provided on its interior surface witha conductive coating, e. g., a layer of a metallic salt, in order toreduce the effect of wall charging.

The envelope or vessel lies symmetrically around the axis of a laminatedmagnetic structure havinga central flux path provided by a laminated.annular core l3. This core is supported at its extremities by attachmentto the central portions of opposed pole pieces l4 and I5 which havecircular areas It and I! and tapered annular areas or pole faces [8 andi9. These poles pieces are, in turn, supported by a rectangular frame 28of laminated iron which surrounds and extends transversely to envelopel0.

The ends of core [3 are separated from pole pieces [4 and i5 by narrowgaps 2! and 22, filling which are disc-shaped elements 23 and 24 ofnonmagnetic, insulating material. Gaps 2i and 22 are so proportioned asto cause core I3 to saturate at a predetermined level of the magneticflux passing through it. The annular faces l8 and IQ of pole pieces Mand I5 each have adouble taper as shown, the purpose of thisconfiguration being explained at a later point.

The magnetic structure is excited by means of a pair of series-connectedcoils 26 and 21 which surround pole pieces l4 and I5 and which may beenergized in such a manner as to provide sinusoidal time-varying flux inthe magnetic circuit.

Electrons produced within envelope W are affected in two ways by thevariations in magnetic flux thus obtained. In the first place, since themagnetic flux traversing core it links the circular path provided byenvelope Ill, any variation of such flux necessarily produces an eddyfield tending to accelerate electrons projected along such path. Inthis-latter respect, the apparatus is comparable to a transformer havinga secondary winding comprising a circular path along which the variouselectrons are accelerated. In general, although the voltage per turn insuch a transformer may be low, within a practically obtainable range offlux variation, the electrons can be made to achieve very high energies,e. g. several million electron volts, because of the tremendous numberof turns which they may execute during a single cycle of magnetic fluxvariation. In addition to the acceleration produced by flux linking theelectron path, the flux produced by annular pole faces !8 and I9 in theregion of the electron orbit tends to cause the electrons to follow aninwardly spiraling path. It has been shown that by a proper design ofthe magnetic structure the centripetal force produced by the magneticfield existing at the electron orbit may be caused to balance thecentrifugal tendencies of the accelerated electrons. In general, thisrequires that the following relationship be satisfied:

where 4 is the flux included in the electron orbit, 1'0 is the radius ofthe orbit, and Hr is the field strength at the orbit. This equationindicates that the flux must be twice as strong as that which would beproduced by a homogeneous field having an intensity Hr and extendingover the entire area enclosed by the orbital electron path. Thiscondition may be realized by making the reluctance per unit ofcross-sectional area of the magnetic path at the electron orbitgreaterby an appropriate amount than its average reluctance within the orbit.In order to maintain the desired proportionality between the enclosedflux and the guide field, i. e., the field Hr, at all times during anaccelerating period, one may adjust the air gap existing between polefaces l8 and I9 and the insulation-filled gaps 2i and 22 to theappropriate value. It is readily practicable to control the dimensionsof the gap from point to point over the pole faces in such a fashion asto effect the balanced relation of guide field and enclosed flux whichis desired for the purpose specified above and which is furthernecessary for radial and axial stability of the electron orbit. This maybe done, for example, by a construction'such as that shown in Fig. l inwhich pole faces l8 and i9 are doubly tapered. In general, asatisfactory compromise between the demands of radial and axialstability of the electron orbit may be obtained by making IIr, the guidefield intensity between pole faces 18 and I9, proportional to and setforth in U. S. Patent No. 2,394,070, granted February 5, 1946, to D. W.Kerst. A

. suitable field profile for radial and axial stability alongappropriate radii in the plane of the orbit between pole faces l8 and i9is shown by curve A in Fig. 2 wherein logior is plotted along a linearabscissa scale and logmB (B is the magnetic induction and is related toHr through a permeability factor, t) is plotted along a linear ordinatescale. It will be readily appreciated that the slope of curve A at anypoint represents the value of the local index, n, at that point. Dottedlines B and C represent a value of n of 1 and respectively. An extractordevice 28, shown sche matically in Fig. 1, for directing the acceleratedelectrons from vessel l0 may be located in the region of line D withinthe n=1 region as will be more fully described hereinafter. Target 29,which may consist of a suitable material such as tungsten supportedwithin envelope Iii as shown schematically in Fig. 1, may be locatedwithin the region of line E. The inner and outer edges of pole pieces l8and B9 are represented by lines F and G, respectively.

When all the conditions specified in the foregoing are fulfilled,electrons introduced into vessel ill in a period when the magnetic fieldis in-- creasing may be expected to be drawn into the particular orbitin which a balance of centripetal and centrifugal forces exist and to becontinuously accelerated along such orbit as long as the magnetic fieldincreases in value. In Fig. 3, curve A may be considered. to representthe time variations of magnetic field or alternatively the timevariation of current in energizing windings 2t and 27. With thisrepresentation, electrons may be conveniently introduced into chamber itby energization of source ll, as will be more fully describedhereinafter, at a time represented by line a. Assuming that the peakvalue of the magnetic field is suiiiciently high, a total energy on theorder of several million electron volts may be acquired by theaccelerated electrons in a small fraction of a second.

It may be noted, however, that when an. electron has obtained a velocitycorresponding to an energy level of about two or three million electronvolts, it is already within about three per cent of the velocity oflight. Accordingly, further gain in energy results primarily in an incl"'e in the mass of the affected electrons and only insignificantly infurther gain in electron velocity, this result being consistent with theEinstein massenergy equivalence formula. Accordingly, electrons whichhave obtained a velocity within a few per cent of the velocity of lightwill gyrate with a relatively constant periodicity or at a relativelyfixed frequency, provided they can be confined to an orbit of relativelyfixed radius. As is pointed out in the aforementioned Pollock andWestendorp Patent 2,485,409, this consideration makes it possible toimpart further energy to the electrons by means of a localized electricfield of fixed frequency acting repetitively on the electrons as theycontinue their gyrations within the envelope [0. A suitable device forobtaining a properly localized electric field of this nature maycomprise a resonator structure 30, shown schematically in Figs. 4 and 8,which can be conveniently constructed as a sector of vessel I0. Variousforms of resonator structure which may be advantageously adapted to suchpurposes are disclosed in the copending application Serial Number691,293, filed August 1'7, 1946, now U. S. Patent 2,553,312, by AnatoleM. Gurewitsch and assigned to the assignee of the present invention. Bythis means extremely high energy levels can be reached through amechanism which avoids difiiculties associated with any attempt toachieve corresponding energy levels by magnetic acceleration alone. Withthis in mind, the apparatus of Fig. l is so constructed that saturationof core l3 occurs after a sufficient acceleration of the electrons hasbeen obtained by magnetic means, such as at a time indicated by line bof Fig. 3, and then resonator 39 is brought into play at a timerepresented by line 0 to produce a high frequency voltage represented bycurve B. However, the guide field produced between pole faces [8 and I9continues to increase as a result of continued energization of coils 26and 21 in order that the accelerated electrons may still be confined tothe desired orbit. When the electrons have reached a desired energylevel at or before the magnetic field represented by curve A of Fig. 3has reached its maximum value, e. g., such as the time indicated by lined of Fig. 3 resonator 30 may be deenergized whereby, since the force ofthe localized electric field tending to impart further energy to theelectrons has been removed, subsequent increase of-the guide field willcause the electrons to spiral inwardly where they may strike target 29of Fig. 1. If it is desired, however, resonator 30 need not bedeenergized until after the magnetic field has passed over its maximumvalue. Thus, resonator 30 may be deenergized at a time indicated by linee of Fig. 3 whereby, since the guide field decreases after the force ofthe localized electric field has been removed, the electrons will spiraloutwardly. Such an expedient is necessary .in the attainment of some ofthe objects of the present invention, as will appear presently.

The operational correlation of the apparatus hereinbefore described maybest be understood by reference to Fig. 4 which shows diagrammaticallythe accelerating structure as a wholein combination with schematicallyillustrated excitation equipment. In this figure, parts which have beendescribed heretofore bear reference characters corresponding to those bywhich the have already been identified.

Referring particularly to Fig. 4, there is shown a power source 3|adapted to supply excitation voltage of the desired frequency, e. g., 60cycles, to windings 2B and 21 by which the magnetic system of theaccelerator is energized. A bank of capacitors represented by capacitors32 may be connected as illustrated between source 3| and windings 26 and21 for the purpose of power factor correction. A second power source 33,which is assumed to be appropriately connected to charged particular orelectron source I I of Fig. l, and which may be an intermittentlyenergized circuit of the type described in the Pollock and WestendorpPatent 2,485,409, serves to inject electrons into vessel W atappropriate intervals correlated with the cyclical reversals of themagnetic field as heretofore mentioned. This correlation may be obtainedby means of a saturable strip 34 which saturates at a desired time andcauses a triggering voltage to be induced in a winding 35 connectedtosource 33. ,A high frequency power source 36, which may consist, forexample, of an electronic oscillator, when energized supplies highfrequency potential through transmission line conductors 3! and 38 toresonator 30. Means for initiating the energization of source 36 maycomprise a saturable strip 39 with an associated winding 40 connected toa grid controlled gaseous discharge tube switch 4|, such as thatdescribed in the aforementioned Pollock and Westendorp Patent 2,485,409.

In order to deenergize source 36 after the electrons have beenaccelerated for a desired time, a voltage, in proper time phase with thevoltage of source 3| and derived through a phase shifter 42 connected tosource 3|, may be applied to discharge tube switch 41. The electrons maythen be ejected from the accelerator apparatus by directing them intoand through an electric field supplied by extractor device 28, shownschematically in Fig. 1. This electric field may be supplied by a highvoltage transformer 43, one end of the secondary winding 43' of which isconnected through a conductor 44 into a side arm 45 in envelope I3 whichsupports extractor device 28. The other end of secondary winding43 isconnected to ground as shown. The primary winding 46 of high voltagetransformer 43 is connected through a phase shifter 41 to source 31 inorder that the electric field through which the electrons are directedfor ejection from the accelerator apparatus may at all times have anintensity proportional to the intensity of the magnetic guide field.

Heretofore, it has been considered that operation of apparatus such asthat hereinbefore described would prevent the successful extraction of acollimated beam of accelerated electrons. This belief has arisen fromthe fact that, since the electrons are accelerated in a stable orbitclosely confined by the vertical and radial restoring forceshereinbefore discussed, they could only be ejected by destroying theso-called betatron relationship of Equation 1 or by destroying therelationship between the accelerating force applied by high frequencyresonator 30 and the magnetic guide field. If either such expedient isemployed, it will be readily appreciated that the electrons will notemerge from the stable orbit at one particular point along thecircumference thereof but instead will have a great number of points ofemergence spatially distributed around the circumference of the stableorbit. This effect is similar to the effect one would expect when wateris placed upon the circumference of a rotating wheel. Consequently, itmay be seen that a collimated beam of electrons may not be ejected fromsuch accelerator apparatus without additional considerations. Accordingto the present invention, highly efficient collimation and ejection ofthe acceleratedelectrons are facilitated i by the employment of thehereinbefore mentioned extractor device 28.

Referring particularly now to Figs. 5, 6 and '7, wherein portions shownbefore are identified by identical reference characters, there is shownin detail the extractor device 28 disposed within the interior ofenvelope I El. Extractor device 28 comprises a pair of spaced apartcurved support members 48 and it attached to C-shaped frame members t,56, 5t and 5! by any means such as soldering. Extending across the inneredges of and between support members 48 and s9 is an electrode 52 whichmay consist of a thin strip of nonmagnetic material, e. g., molybdenumabout 0.002 thick, lapped over and secured to support members it and 49by any convenient means such as spot welding. Electrode 52 may bemaint-ained at ground potential by connecting to it a suitable conductor(not shown) introduced through aside arm into envelope In in a mannerwell known to those skilled in the art. Opposed. to and radially spacedfrom electrode 52 is a curved T-shaped high voltage electrode 53supported from C-shaped frame memberstfl and 5! by means of insulators 55 and'55. Insulators 54 and 55 may comprise conductive stems 56 and 57supported with adjacent ends spaced apart by means of an insulatingspacer member 58. For the purpose of reducing the likelihood ofsparkover between high voltage electrode 53 and the C-shaped supportmembers which are grounded, shielding cups 59 and 66 are provided at thelower and upper ends of insulating spacer 58 respectively. Conductors 5Sand 51 may be attached to the C-shaped support members and high volt ageelectrode 53, respectively, by any convenient means such as soldering.In order to provide support and accurate desired radial positioning ofstructure 28, end studs BI and 62 are attached, such as by soldering, tothe respective ends of curved support members 48 and 49. The outer endsof studs 65 and 62 bear against the inner periphery of envelope it andare curved to conform generally thereto. The portions of studs iii and62 which traverse the radially extending space between electrodes 52 and53 are provided with openings 63 and 64 to permit accelerated electronsto enter and emerge from structure 28, as. will be more fully describedhereinafter. For the purpose of conducting high voltage to electrode'53and also for the purpose of providing support for structure 28,conductor Mi, in the portion entering envelope l0 shown in Fig. 5, maycomprise a rigid conductive rod having a diametrically extending slot 65in its inner end which makes press fit with electrode 53. Conductor itmay he seal-ed within side arm 45 in a manner well known to thoseskilled in the art.

In view of the foregoing, it will be readily understood how a beam ofhigh energy electrons may be successfully ejected from the acceleratorapparatus. Let use assume that electrons have been injected from sourceH and accelerated as hereinbefore described, the direction of travel ofsuch electrons being counterclockwise along the stable orbit asindicated in Fig. -5. If, by a properly timed voltage correlated withsource 3! of. Fig. l and supplied through phase shifter 42 to dischargetube switch il, source 36 is then deenergized after the maximum of themagnetic field represented by curve A of Fig. 3 has been reached, theelectrons will spiral outwardly toward ejector structure 28 because themagnetic guide field decreases with time after the accelerating force ofsource 36 has been removed. As

64, and traverse the electric field between electrodes 52 and 53. Itwill be apparent that, if the potential applied to electrode 53 ispositive with respect to the ground potential of electrode 52, theelectrons will'be attracted toward electrode 53 and hence the pitch ofvthe spiral paths, followed by the electrons before they enteredstructure 28, will be increased. Thus, by the proper application of anelectric field, the electrons may be caused to emerge from opening 63with a trajectory which directs them from envelope l0 shortlythereafter.

In order to secure successful ejection of the electrons with maximumvefficiency, there are several important considerations which must beobserved. In the first place, extractor device 28 must have the properradial disposition within envelope In so that it may properly interceptthe spiraling electrons. As has been mentioned heretofore in connectionwith Equation 2 and Fig. 2, the electrons traverse an axially andradially stable orbital path when the local index, n, is between 0and 1. However, when the local index, n, is equal to or exceeds 1 invalue, the electrons are radially unstable inasmuch as the radialrestoring forces produced by the guide field Hr are insufiicient to keepthe electrons within a defined stable orbit. Therefore, it is desirableto position extractor device 28 within the n=1 region, as indicated inFig. 2, in order to take advantage of the radial instability of theelectrons within that region and thereby enhance the effectiveness withwhich the electric field appearing between electrodes 52 and 53 mayincrease the pitch of the spiral paths followed by the electrons. It mayalso be shown that the following relation must be observed:

where e is the radius of curvature of the extractor, Hr is the magneticfield intensity where n='1, m-1 is the radius where n=1, He is themagnetic field intensity at a point between electrodes 52 and 53, and Eis the electric field intensity between electrodes 52 and 53. Thisrelation means that the radius of curvature of I the extractor device 28must be somewhat larger than the radius of the n=1 region. It has beenfound that ordinarily E is of the order of 10% of H and, therefore, 6must be proportionally larger than m to maintain the above relation.Commonly, ejector structure 28 is arranged so that the portion ofelectrode 52 adjacent opening 64 lies approximately on the radiallyinward edge of the n=1 region indicated in Fig. 2.

Equation 3 also shows the dependency between the electric fieldintensity E and the intensity of the magnetic field. If the electronsare to be successfully ejected at any desired energy level, therefore,the intensity of the electric field E must be correlated with theintensity of the magnetic guide field Hr at the time the electrons enterthe ejector structure 28 in such a manner as to maintain Equation 3.This is accomplished according to the present invention as explained inconjunction with Fig. 4 by connecting con! ductor 44 so as to provide ahigh voltage to electrode 53 which isdependent in time upon the voltageof source 4|. The field E thus supplied between electrodes 52 and 53 isnot strictly electrostatic according to common terminology since itvaries in time, but, insofar as the electrons so small as to beinsignificant.

are concerned, they experience essentially the force of an electrostaticfield because they are spiraling with a velocity approximately that oflight, and hence any variations in time of the field E while theelectrons are traversing it are Therefore, the term essentiallyelectrostatic may be employed as descriptive of the field E.

In Fig. 8, there is shown a view which illustrates in schematic fashiona disposition of the various portions of the apparatus hereinbeforedescribed which may be suitably employed in connection with the presentinvention. Electron source H may be positioned above or below the planeof the electron orbit and within the region where n is between and 1 inorder that electrons injected into the apparatus may not strike theextractor device 28 prematurely. Instead of being supported by means ofa side arm 12, as shown, electron source H may with eflicacy besupported above or below the plane of the orbit by attaching it throughinsulators (not shown) to extractor device 28. Extractor device 28 ispositioned with its leading edge (adjacent open- .ing 64) approximatelyat the inward radial edge of the 11:1 region with a magnetic guide fieldhaving a general configuration similar to that illustrated in Fig. 2.Stud 62 is of such a length as to assure this positioning while stud 'BIis of such a length as to bring the lagging edge of extractor device 28(adjacent opening 63) to a somewhat larger radius. With the properelectric field intensity between electrodes 52 and 53 and with a desiredazimuthal positioning of extractor device 28 as indicated, theelectrons, spiraling outward after resonator 30 has been deenergized toenter extractor device 28, may be expected to have the pitch of thespiral paths, which they had followed therebefore, increased in such amanner as to cause them to emerge from the envelope H) as illustrated bylines 65. In

order to permit efiicient emergence of the electrons from the envelopeHi, there is provided a circumferential port 67 having a terminatingelectron permeable window 68 secured thereto in a manner well known tothose skilled in the art.

,The accelerator apparatus may also be employed in accordance with theinvention to generate high energy X-rays by positioning X-ray target 29along a smaller radius than the radius of the stable orbit anddeenergizing resonator 30 in a particular cycle of magnetic field beforethe magnetic field has reached a maximum value. Thereupon the electronsspiral inwardly, strike X-ray target 29, and emerge through window 68 asof leg (Fig. 8) which isa portion of magnetic structure 25. Theordinates of the graph represent the voltage applied between extractorelectrodes 52 and 53 or the extractor voltage. Curve A shows thevariation of maximum beam intensity with extractor voltage when theleading edge of extractor device 28 (adjacent opening P64) is positionedat the innermost edge of the 11:1 region so that extractor device 28would be, except for, its slighter larger curvature, positioned withelectrode 52 lying along the innerresponding reference characters.

most radius of the n=l region. Curve B represents the variation ofmaximum beam intensity with extractor voltage when extractor device 28is positioned as in curve A except the whole extractor device is at aslightly larger radius. Curve C represents the same variation whenextractor device 28 is positioned as in curve A except that thelaggingedge (adjacent opening 63) is tilted outward slightly. These curvesclearly illustrate that the pitchof the spiral paths followed by theelectrons may be readily increased by increasing the voltage betweenelectrodes 52 and 53. Also, the pitch may be increased by tilting thelagging end of the extractor device slightly outward. Moreover, as curveB indicates, with extractor device 28 at a radius slightly larger thanthe innermost radius of the n=l region, effective extraction may beobtained. However, it has been found that, with the extractor device ata slightly smaller radius than the innermost radius of the n=l region,i. e., where n is between 0 and 1, suitable extraction cannot beobtained.

In Fig. 10, there is shown alternative circuitry for assuring that thevoltage applied between electrodes 52 and 53 is in proper time relationwith the variations of the hereinbefore mentioned magnetic field. It maybe observed that the current in series-connected windings 26 and 21,which excite, magnetic structure 20, is proportional to the variationsof magnetic field. Hence,

the desired variable voltage between electrodes 52 and 53 may beobtained by means of a current transformer II connected in circuit withwindings 26 and 21 to supply a step-up high voltage transformer T2 andconductor 44 which is connected to electrode 53. The primary winding ofcurrent transformer Il may comprise the conductor I3 interconnectingwindings 26 and 27 and capacitor 32, while the secondary winding 14 oftransformer 1| may comprise a few turns positioned about conductor 73.Since electrode 52 is grounded the high voltage end of secondary windingof transformer 72 may be connected through conductor 44 to electrode 53,While the other end of winding I5 may be connected directly to ground asindicated. To secure the proper phasing of the voltage betweenelectrodes 52 and 53 with respect to the varying magnetic field, thereis provided a variable capacitor 115 connected across the primarywinding 17 of transformer 12. The magnitude of the voltage appliedbetween electrodes 52 and 53 may be adjusted by means of a var ableresistor '18 connected in parallel with variable capacitor 78 andprimary winding 71 of transformer 72. In some instances, in order tosecure the proper phasing as hereinbefore meniggonzgi it maytbe Tfoundnecessary to replace 1 e capaci or 6 with, (not shown) a variableinductor It is also within contemplation of the I invention that thevoltage applied betwee ie lz trodes 52 and 53 may be in the form of. apulse having a desired magnitude proportional to the ntensity of themagnetic guide field at the time t is applied. Suitable circuitry foraccomplishing such a purpose is shown in Fig. 11 wherein elementsalready shown and described bear cor- Pulses of the desired nature maybe obtained by charging a capacitor "I9 to a voltage proportional to theintensity of the magnetic guide field and discharging such capacitorthrough a grid-controlled gaseous discharge device 80 and a resistor 8!,Means for so charging capacitor 19 comprise an auto-transformer 82 fordetermining the magnithrough resistor 8 I.

i l tude of the pulse connected across source 3! as indicated. Themovable contact- 83 of autotransformer 82 is connected to the primarywinding 84 of a high voltage step-up transformer 85.

One end of the secondary Winding 85 of transformer 35 is connected toground while the other end is connected to the plate 8? of a dioderectifier device 88. The filament 8-9 of rectifier device 88- isconnected through a current-limiting re-' sistor 90 to capacitor is andthe plate 9| of discharge device 80. Since source 31 energizes windings26 and 21, which in turn excite magnetic structure 20 to provide for themagnetic field within the accelerator structure, it may be seen that thecapacitor i9 is charged to a potential proportional to the intensity ofthe magnetic guide field. Resistor @I is grounded, as shown, to completethe. charging circuit from secondary winding 86 of transformer 85through rectifier device 88, resistor 953, capacitor 19 and resistor Inorder to provide a desired bias for discharge device ditthere is shown asource of direct current :potential S2 connected in series with a gridleak resistor E33 between the grid 94 and filament 95 of dischargedevice 8! Means for triggering device til at. a desired time todischarge capacitor til may comprise a saturable strip 96 disposedbetween pole pieces [4 and i5 and havingawinding 9i inductively linkedthereto. One

end of winding 91 is connected through a capacitor 93. to grid 94 ofdischarge device 80 while the other end. is connected to ground. Thetime at which strip 96 saturates to cause a voltage pulse: to begenerated in winding 97 for triggering discharge device it may becontrolled by current carrying winding 99 connected in series with asource of direct current potential Hit, a variable resistor Ill-E and areactor I02. Winding 99 is inductively linked with saturable strip 96and thus may be employed to determine the time at which strip 96saturates. Variable resistor Iili has the function of limiting thedirect current flowing through winding 99 while reactor I02 preventswinding 99 from loading winding 9? when. a voltage is generated thereinby the saturation of strip 98. As will now be readily understood,discharge device 80 may be triggered by. av voltage supplied fromwinding 91 at a desired time whereby capacitor is will discharge Thiscauses a voltage to appear across resistor 81' and, if electrode 53 isconnected through conductor fi l to resistor- 81 as indicatediavoltagepulse which has a magnitude proportional to the intensity of themagnetic field and which may be arranged to occur at a desired time thecycle of magnetic field is applied be- .tween electrodes 52 and 53.

In Figs. 12 and 13, there is shown alternative structure for anextractor'device. Herein extractor device !28- comprises a curved member)3 of insulating material having a longitudinal channel m4 within whicha high voltage'electrode: ['53 is disposed. High voltage electrode I 53is supported within channel l-M'by means of a conductor M4 which extendsthrough a hole in member Hi3 and screws into a nut IE5 attached toelectrode I53 by any means such as soldering.

Ashoulder its may be provided on conductor M4 for determining the properradial position or" high voltage electrode i53 within channel H14.Electrode I52, which may be maintained at of thin wires I01 which extendaround member I03 to be attached at both ends to electrode 52 by anyconvenient means such as spot welding. An end plate [08, having anopening 1% therein to permit the entrance of electrons into extractordevice I28, may be attached to member I83 by means of screws HIS.Shoulders Hi and H2 may be provided at the respective ends of member 03for the purpose of bearing against envelope lit to secure the properradial positioning of structure 28.

As has been. mentioned heretofore in connection with Fig. 5, electrode52 should consist of a thin strip of nonmagnetic material, e. g.,molybdenum about 0.002 thick. Electrode 52 should be as thin. aspermissible in view of structural limitations in order to keep theelectrons from striking the leading edge of electrode 52 duringextraction. Moreover, all of the metallic material utilized withinextractor device 28 should be nonmagnetic, e. g., molybdenum orstainless steel, etc., so. that eddy currents induced by the changingmagnetic field may be minimized. The radial spacing between high voltageelectrode 53 and electrode 52 should be greater than the spac ng betweensuccessive orbits of the electrons within the region where: extractordevice is positioned, but yet not great enough to prevent theestablishment of the required high voltage gradient. Any solderingemployed in connection with the construction of extractor device 28should utilize a high melting point solder in order toobviatedestruction of the joints by heating due to eddy currents.

While my invention has been described by reference to particularembodiments thereof, alternative constructions will rea'dily occur tothose skilled in the art. I, therefore, aim in the appended claims tocover all. such equivalent embodiments as may bev within the true spiritand scope of the foregoing description.

What I claim as new and. desire to secure by Letters Patent of theUnited States is 1'. A system for producing a beam of high energyelectrons comprising magnetic induction accelerator means foraccelerating electrons in an orbital path including means providing analternating magnetic flux which links said path to accelerate saidelectrons and means providing an alternating magnetic guide field which:traverses said path to constrain said electrons thereto, high frequencyelectrodes for imparting further energy to said electrons in saidorbital path after acceleration to approximatelythe velocity of light bysaid alternating flux, oscillator means for energizing said electrodesin timed correlation with saidalternating field, means for deenergizingsaid oscillator means and said electrodes in timed correlation withsaidalternating field whereby said electrons may be diverted from saidorbital path into spiral paths, and spaced extractor electrodes adjacentsaid orbital path for increasing the pitch of said spiral electronpaths, at least one of said extractor electrodes being connected to analternating voltage source.

21 A system for producing a beam of high energy electrons comprisingmagnetic induction accelerator means for accelerating electrons in anorbital path including means providing an alternating magnetic fluxwhich links said path to accelerate said electrons and means providingan alternating magnetic guide field which traverses said path toconstrain said electrons thereto. high frequency electrodes forimparting further'energy' to said electrons in said orbital path 'fieldwhereby said electrons maybe diverted from said orbital path into spiralpaths, and spaced extractor electrodes adjacent said orbital path forincreasing the pitch of said spiral electron paths, at least one of saidextractor electrodes being connected to an alternating voltage sourcehaving a time phase correlated with said alternating magnetic guidefield.

3. A system for producing a beam of high energy. electrons comprisingmagnetic. induction accelerator means for accelerating electrons in anorbital path including means providing an alternating magnetic fluxwhich links said path to accelerate said electrons and means providingan alternating magnetic guide field which traverses said path toconstrain said electrons thereto, high frequency electrodes forimparting further energy to said electrons in said orbital path afteracceleration to approximately the velocity I related in time andamplitude with said alternating magnetic field.

4. A system for producing a beam of high energy electrons comprisingmagnetic induction accelerator means for accelerating electrons in anorbital path including means providing an alternating magnetic fluxwhich links said path to accelerate said electrons and means providingan alternating magnetic guide field which traverses said path toconstrain said electrons.thereto, high frequency electrodes forimparting further energy to said electrons in said orbital path afteracceleration to approximately the velocity of light by said alternatingflux, oscillator means for energizing said electrodes in timedcorrelation with said alternating field, means for deenergizing saidoscillator means and said electrodes in timed correlation with saidalternating field whereby said electrons may be diverted from saidorbital path into spiral paths, and electric field producing means forejecting said electrons from said orbital path, said last-named meansbeing in timed correlation with said alternating magnetic field toproduce an electric field having an amplitude and phase dependent uponthe variations of said alternating magnetic field.

5. Electron accelerator apparatus comprising means for acceleratingelectrons in a stable orbital path including means providing analterhating magnetic flux which links said path to accelerate saidelectrons and means providing an alternating magnetic guide field whichtraverses said path to constrain said electrons thereto, said last-namedmeans being arranged to satisfy the proportionality where H1- is thefield strength of said magnetic guide field, r is the radius at which Hris measured, and n is an exponent having a value between 0 and 1 in thestable orbit region, high frequency electrodes coupled to said path forimparting further energy to said electrons, a source of high frequencyoscillations connected to said electrodes, a discharge tube switchconnected to energize said source at a predetermined time and todeenergize said source when said electrons have been accelerated to adesired energy level whereby said electrons will be diverted from saidstable orbit into spiral paths, and spaced extractor electrodespositioned adjacent said path to provide an azimuthal essentiallyelectrostatic field which saidspiralin relectrons will traverse, one ofsaid electrodes having its leading edge positioned within a region ofalternating magnetic field where n=1 and another of said electrodesbeing connected to a source of high voltage correlated in time with thevariations of said alternating magnetic field.

6. A system for producing a beam of high energy charged particlescomprising magnetic induction accelerator means for accelerating chargedparticles in an orbital path including means providing a time-varyingmagnetic flux which links said path to accelerate said charged particlesand means providing a time-varying magnetic guide field which traversessaid path to constrain said charged particles thereto, high frequencyelectrodes for imparting further energy to said charged particles insaid orbital path after acceleration to a desired velocity by saidtime-varying flux, oscillator means for energizing said electrodes intimed correlation with said time-varying field, means for diverting saidcharged particles from said orbital path into spiral paths, and spacedextractor electrodes adjacent said orbital path for increasing the pitchof said spiral charged particle paths, at least one of said extractorelectrodes being connected to a source Of time-varying voltage.

'7. A system for producing a beam of high energy charged particlescomprising magnetic induction accelerator means for accelerating chargedparticles in an orbital path including means providing a time-varyingmagnetic flux which links said path to accelerate said charged particlesand means providing a time-varying magnetic guide field which traversessaid path to constrain said charged particles thereto, high frequencyelectrodes for imparting further energy to said charged particles insaid orbital path after acceleration to a desired velocity by saidtime-varying fiux, oscillator means for energizing said electrodes intimed correlation with said time-varying magnetic field, means fordiverting said charged particles from said orbital path into spiralpaths, and electric field producing means for ejecting said chargedparticles from said orbital path, said last-named means being in timedcorrelation with said time-varying magnetic field to produce an electricfield having an amplitude and phase dependent upon the variations ofsaid time-varying magnetic field.

8. Charged particle accelerator apparatus comprising means foraccelerating charged particles in a stable orbital path including meansproviding a time-varying magnetic flux which links said path toaccelerate said charged particles and means providing a time-varyingmagnetic guide field which traverses said path to constrain said chargedparticles thereto, said 15 last-named means being arranged to satisfythe proportionality.

H gi;

Where Hr is the field strengthof said magnetic uidefield, 1' is theradius at which H1 is measured, and n is an exponent having a valuebetween 0 and 1 in the stable orbit region, high frequency electrodescoupled to said path for imparting further energy to said chargedparticles', a source of high frequency oscillations connected to saidelectrodes, means fordiverting said charged particles from said stableorbital path. into spiral paths after said charged particles have been.accelerated to a desired velocity, and spaced extractor electrodespositioned adjacent said path to provide an azimuthal essentiallyelectrostatic field which said spiraling charged particles traverse, oneof said electrodes having its leading edge positioned within a region oftime-varying magnetic field where n=1 and another of said electrodesbeing connected to a source of high voltage correlated in time with thevariations of said time-varying magnetic field. v

9. Charged particle accelerator apparatus comprising means foraccelerating charged particles in a stable orbital path includingproviding a time-varying magnetic flux which links said path toaccelerate said charged par ticles and means providing atime-varyingmagnetic guide field which traverses said path to constrainsaid charged particles thereto, said lastnamed means being arranged tosatisfy the proportionality H ongwhere Hr is the. field strength of saidmagnetic guidefield, r is the radius at which Hr is measured, and n isan exponent having a value between 0 and 1 in the stable orbit region,high frequency electrodes coupled to said path for imparting furtherenergy to said charged particles, a source of high frequencyoscillations connected to said electrodes, means for diverting saidcharged particles from said stableorbital path into spiral paths aftersaid charged particles have been accelerated to a desired velocity, andspaced extractor electrodes positioned adjacent said path to provide anazimuthal essentially electrostatic field which said spiralin chargedparticles will traverse, one of said extractor electrodes having itsleading edge positioned Within a region of time-varying magnetic fieldWhere nsl'ightly less than 1 and another of said electrodes beingconnected to a source of high voltage correlated in time with thevariations of said time-varying magnetic field.

10. A system for producing a beam of high energy charged particlescomprising magnetic induction accelerator means for accelerating chargedparticles in an orbital path including means providing a time-varyingmagnetic flux which links said path to accelerate said charged particlesand means providing a time-varying magnetic guide field which traversessaid path to constrain said charged particles thereto, means increasingthe pitch of said spiral charged particle paths, at least oneof saidextractor electrodes being connected to a source of time-varyingvoltage.

HERBERT C. POLLOCK.

References Cited in the file of this patent unrrsp STATES'PATENTS NumberName Date 1,9483% Lawrence Feb. 20, 1934 2,193,662 Penney Mar. 12, 19402,394,070 Kerst Feb. 5, 1946 2,533,859 Wideroe Dec. 12, 1950 2,553,305Dickinson May 15, 1951 2,553,312 Gurewitsch May 15, 1951 OTHERREFERENCES Review' of Scientific Instruments Design Study for a Ten Bev.Magnetic Accelerator, Brubeck, vol. 19, No. 9, September 1948, pages5454351.

